1. Field of the Invention
The present invention relates to an optical module, and more particularly to a TO-CAN type optical module employing a TO-CAN type package.
2. Description of the Related Art
An optical module plays an important role in exchanging optical and electrical signals. As the demand for a faster data transmission rate has been increased rapidly, an optical module capable of transmitting a wide bandwidth of frequencies is needed. The optical modules typically include transmitters for converting an electrical signal into an optical signal for subsequent transmission and receivers for converting the transmitted optical signal into an electrical signal.
Alternatively, a transceiver maybe used as an optical module. In the transceiver, the electrical signal and the optical signal are interchanged using a laser diode (LD) and a photo diode (PD). These LD and PD are installed separately, then sealed in a single package. A TO-CAN type package is most economical and thus used widely as an optical module package.
FIGS. 1a and 1b are schematic views of a conventional TO-CAN type package. In particular, FIG. 1a is a plan view of the conventional TO-CAN type package, and FIG. 1b is a cross-sectional view taken along the line A–A′ of FIG. 1a. FIGS. 2a and 2b illustrates a connection between the TO-CAN type package of FIG. 1 and a printed circuit board (PCB) 40 for signal processing. In FIGS. 1a, 1b, 2a and 2b, optical components mounted on a stem in the TO-CAN type package are omitted and only the stem and pins are shown. FIGS. 3a and 3b are graphs illustrating a high-frequency property of a conventional TO-CAN type package (T056), and more specifically, FIG. 3a represents a gain property, and FIG. 3b represents a matching property. In the drawings, a non-described reference number 1 denotes a cap.
With reference to FIGS. 1a, 1b, 2a, and 2b, the conventional TO-CAN type package comprises a stem 10, which is provided with optical components (not shown) such as a LD and matching resistors for transmitter and a PD and TIA (Transimpedance Amplifier) for receiver mounted on its upper surface, signal-carring pins 20, and a ground pin 30 passing through the stem 10 so that the signal-carrying pins 20 and the ground pin 30 are connected to the optical components.
Note that the characteristic impedance of each of portions 21 sealed with the glass in the TO-CAN type package is very low, i.e., 20Ω, and the inductance of each of the pins 20 and 30 protruding from the stem 10 is high. Accordingly, a high frequency signal of 50Ω in transmission must be attenuated in the TO-CAN type package. When the electrical signal of the TO-CAN type package is transmitted to an external substrate at a data transfer rate of less than 2.5 Gb/s, the best way to enable communication is to shorten the length of the pins 20 and 30 as much as possible. Particularly, at a date transfer rate of more than 10 Gb/s, a very small distance of 1 mm between the stem 10 and the printed circuit board (PCB) 40 degrades the signal severely when the TO-CAN type package is mounted on the PCB 40, which is manufactured with a signal line with a characteristic impedance of 50Ω. Accordingly, it is very important to attach the stem 10 close to the printed circuit board 40.
In a system provided with a TO-CAN type optical module, since an optical axis of a transmitter coincides with an optical axis of a receiver, pins of the TO-CAN type optical module require a uniform length in consideration of the difference between the positions of the signal-carrying pins of each of the modules (the distances from each of the optical axes) and the variation generated in manufacturing the module. As a result, a signal loss at 10 GHz, which is more than 2 dB, is generated due to the structures of glass-sealed portions and the lengths of the pins (in FIG. 3a), thereby reducing the bandwidth and lowering the matching property of the optical module, as shown in FIG. 3b. 